1. Field of the Invention
The invention relates to a process for the preparation of a substituted cyclopentadiene compound, and in particular to a process that permits excellent control over the selectivity of the substitutions on the resultant substituted cyclopentadiene compound. The substituted cyclopentadiene compound has utility as, for example, a ligand of a metal or transition metal complex used to catalyze a polymerization reaction.
2. Description of the Related Art
Substituted cyclopentadienes and their derivatives have utility as, for example, ligands in metal and transition metal complexes, which complexes can be used as catalysts in a polymerization reaction. The nature of the ligands, including the substituted cyclopentadienyl ligand, on the metal complexes can have a significant affect on the efficiency of the polymerization reaction and the characteristics (for example, average molecular weight) of the polymers prepared from the process. Accordingly, a process for preparing substituted cyclopentadienes in which the selectivity of the substituted cyclopentadienes produced can be meaningfully controlled would be of great benefit.
The preparation of a substituted cyclopentadiene compound is described by Williams et al., J. Am. Chem. Soc., 113, 4843-4851, (1991) (hereinafter "Williams"), the complete disclosure of which is incorporated herein by reference, which details the preparation of isopropyl substituted cyclopentadiene by the reaction of a mixture of aqueous KOH, isopropyl bromide and cyclopentadiene in a molar ratio of 40:5:1, with ADOGEN 464 being used as a phase transfer catalyst. A drawback of this process is that a mixture of tri- and tetraisopropyl cyclopentadiene in a ratio of about 35:65 is obtained. This mixture has to then undergo a separate isolation step in order to obtain the individual compounds.
From Venier et al., J. Am. Chem. Soc., 112, 2808-2809, (1990) (hereinafter "Venier"), the complete disclosure of which is incorporated herein by reference, it is known to prepare di-tert-butyl cyclopentadiene with a selectivity of 90%; however, such high selectivity appears to be the exceptional case, as seen by the remaining examples set forth in Williams. The high selectivity of the preparation of di-tert-butyl cyclopentadiene is primarily due to the fact that a tertiary alkyl, with a high degree of steric hindrance, is being substituted onto the cyclopentadiene ring. In such a case it is the low probability of obtaining the tri-tert-butylcyclopentadiene compound, with its high degree of steric hindrance, which is responsible for the high percentage of di-substituted compounds.
However, a selection mechanism based on the steric effects of the substituent will not be effective in the case of substituents which have less steric hindrance, for instance n- or sec-alkyl groups.
A need therefore exists for a synthetic route to produce substituted cyclopentadiene compounds, which process allows for improved selectivity, control and flexibility over the number and kind of substituents on the cyclopentadiene ring.